When heating operation of an air conditioner causes frost to form at an outdoor heat exchanger arranged within an outdoor unit, defrosting operation, which is a reverse-cycle operation to heating operation, is performed in order to melt the frost. Upon performing defrosting operation, the outdoor heat exchanger functions as a condenser to dissipate heat, and the formed frost is thawed. Melting of the frost produces thaw water, which falls down from the outdoor heat exchanger and is collected as drain water at a base plate arranged below the outdoor unit and discharged from a drain hole provided in the base plate.
When this defrosting operation is performed under such a severely cold environment that the outdoor temperature stays below the freezing point, drain water flew out to the base plate is cooled to freeze before arriving at the drain hole and becomes no longer dischargeable from the drain hole. Frozen drain water gradually grows larger on the base plate, and eventually causes destruction of the outdoor heat exchanger, an outdoor fan or the like. In addition, even when it does not reach to the point that drain water freezes during flowing out, snow or the like blown into the inside of the outdoor unit or on the base plate may hinder discharge of drain water, which results in freezing of the undischarged drain water, which causes destruction of the outdoor heat exchanger or the like.
To avoid such problems, a water heater in which a portion of high-pressure-side refrigerant piping of a refrigerating cycle is arranged above a base plate is disclosed in a prior art document, Japanese Patent Laying-Open No. 2004-218861 (Patent Document 1). Japanese Patent Laying-Open No. 2004-218861 discloses a drain-pan freeze-prevention structure in which refrigerant piping for freeze prevention is routed in a heat-transmittable manner above a drain pan comprised of base plates and located below an air heat exchanger. Further, an evaporator having a structure in which a refrigerant at a high temperature is allowed to pass through the bottom piping of an outdoor heat exchanger in defrosting operation, thereby increasing an amount of heat given to frost on the drain pan to facilitate thawing of frost is disclosed in a prior art document, Japanese Patent Laying-Open No. 58-49878 (Patent Document 2). An air conditioner having a structure in which a drain outlet is provided below an outdoor heat exchanger, and the vicinity of a drain route is heated by a heater or a base plate heater is disclosed in a prior art document, Japanese Patent Laying-Open No. 2005-49002 (Patent Document 3).    Patent Document 1: Japanese Patent Laying-Open No. 2004-218861    Patent Document 2: Japanese Patent Laying-Open No. 58-49878    Patent Document 3: Japanese Patent Laying-Open No. 2005-49002